1. Field of the Invention
The invention relates generally to photographic sound-tracks for motion picture films. More particularly, the invention is directed to motion picture photographic (generally referred to as "optical") sound-tracks that provide four discrete channels (left, center, right and surround--L, C, R, S) while maintaining compatibility with theatre equipment adapted for playing films recorded in the widely used two-track stereo variable area (SVA) format in which the four channels (L, C, R, S) are matrix encoded onto two optical film tracks.
2. Background Art
Optical sound-tracks for motion pictures were first demonstrated around the turn of the century, and since the 1930's have been the most common method of applying sound on film. The principle of operation is to have an exciter lamp which illuminates a narrow slit, perpendicular to the direction of film travel. A lens focusses the image of the slit onto the film sound-track, which runs parallel to the direction of film travel, and lies between the picture and the sprocket holes. Behind the film a photocell or solar cell detects the amount of light being passed through the film, and the current or voltage generated by the cell is amplified and sent ultimately to the theatre loudspeaker(s). The transmission of light through the film is varied by either variations in density (an essentially obsolete technique) or by variations of width, where an ideally transparent varying width of sound-track is situated within an ideally opaque surrounding. This latter type of sound-track is known as "variable area".
The first variable area sound-tracks had one fixed edge, with the other edge a distance apart which varied with the required audio modulation. This type of optical sound-track is referred to as "unilateral". The varying clear width causes the required modulation in light transmission received by the cell. It was realized in the late 1930's that errors in light uniformity along the length of the slit could cause distortion components; for example, a fall-off in illumination at that end of the slit that corresponded to peak modulation level could cause significant second-harmonic disortion. In an effort to reduce this effect and other geometric distortion components, the "bilateral" variable area track was introduced. This format has two modulated edges, identical mirror images around a fixed centerline. This technique is immune to constant-slope slit illumination error, but will develop some slight second and third harmonic distortion components under parabolic light error conditions.
A later development, which is now the standard monaural optical sound-track format, is called the "dual-bilateral" (or "double-bilateral" or "duo-bilateral") sound-track. This format has two bilateral elements within the same sound-track area, thus providing further immunity from illumination non-uniformity errors.
In the mid 1970's stereo variable area (SVA) tracks became increasingly popular, in which two independently modulated bilateral sound-tracks are situated side by side in the same area as the normal monaural (mono) variable area track. A two element solar cell is used in this case to provide two independent output signals, one derived from each of the two bilateral sound-tracks.
With the exception of the Fantasound system used for the motion picture Fantasia in 1941, (which used two interlocked 35-mm films, one with picture and the other carrying only multiple sound-track elements), no multi-channel optical sound-tracks were used commercially prior to the mid-1970's. Several systems have been developed since 1953 which use magnetic stripes to carry multi-channel sound on 35-mm and 70-mm films, but prints of this type cost much more than films with optical sound-tracks. This high cost and various technical difficulties have precluded widespread use of magnetic stripes to achieve stereo sound in motion picture theaters.
However, as early as 1936 workers in the art experimently demonstrated independent modulation of variable area optical sound-track elements in the normal sound-track area to achieve two separate sound channels. One reason that these proposals were not widely adopted at the time was the difficulty of projector modification, resulting from the necessary incorporation of diverging optics and two separate photo-electric tubes. Even more importantly, these proposals suffered from the drawback that the substantially narrower track width for each channel of information resulted in at least a 3 dB decrease in signal-to-noise ratio, making the arrangement impractical for commercial use. Even though a 3 dB increase in film grain noise might be tolerable, optical sound-tracks pick up dirt and scratches, and the fifty percent reduction in track width makes these impulse noises unacceptable. What was missing and remained unavailable until the late 1960's was an effective noise reduction system that would restore the lost signal-to-noise ratio without introducing undesirable audible side-effects.
In the early 1970's Dolby Laboratories introduced its professional A-Type noise reduction system to the motion picture industry. A number of films were recorded with mono sound-tracks encoded with A-Type noise reduction for performance in theatres having the complementary A-Type decoding equipment. Shortly thereafter Eastman-Kodak and RCA began new investigations into two-track 16-mm stereo optical film sound-tracks. Miniature silicon solar cells had now made projector conversion much easier, and additionally, Dolby noise reduction was employed to solve the narrow track noise problem. Dolby Laboratories extended this development to 35-mm film for professional theatrical use; the resulting sound-track format, optical stereo variable area (SVA), employs two independent bilateral variable area tracks each having a width of 0.033", separated by a septum of 0.010". This makes a total width of 0.076", the same as used for the standard mono format. Thus, some compatibility is provided with theatres having only mono equipment.
The two-track SVA format has become very successful commercially. Over four thousand theatres world-wide are equipped to playback such films in stereo and over three hundred films have been released having some or all of the prints in that format. Some films, including such successful films as Star Trek and Star Wars, have been released with all optical sound-track prints using the two-track SVA format.
In recent years, most two-track SVA format films have been produced with four channels of sound (L, C, R and S) matrix encoded onto the two SVA tracks. When played in theatres, the two SVA tracks are applied to a matrix decoder in order to recover the original four channels. Such an arrangement is generally referred to as a 4-2-4 matrix. The particular matrix used, denoted the MP matrix, is a simple symmetrical matrix in which: (1) the left and right channels are recorded onto the respective left and right optical tracks; (2) the center channel is recorded in phase and 3 dB down in amplitude on both tracks; and (3) the surround channel is recorded out of phase and 3 db down in amplitude on both tracks. One major advantage of this format is that 4-channel decoding is not essential, in that a two channel playback will reproduce the center channel between L and R in the same way as a stereophonic phonograph record or an FM stereo broadcast.
On the other hand, a 4-2-4 matrix system has much worse interchannel crosstalk than a discrete channel system. The apparent separation can be improved over and above the theoretical 3 dB between adjacent channels by use of logic techniques, but the separation relaxes to this 3 dB figure between adjacent channels when signals are sent into all channels simultaneously.
Various discrete four-track optical sound-track systems have been proposed over recent years and, a couple of films have been released with one or more experimental prints in such formats. More recently, a four-track discrete optical format has been proposed in which the L, C, R and S tracks lie within the standard 0.076" width and are separated by three septa of about 0.009" each. Compared with a two-track SVA print, the signal-to-noise ratio of each track is degraded by over 4 dB, requiring severe electronic noise reduction techniques to compensate the grain noise build-up (and consequently placing greater demands on accuracy of frequency response, etc.), and special print processing to reduce the film's pick up of dirt and scratches.
None of the discrete optical multi-track systems in the prior art provide a format compatible with the two track SVA format. Due to the economics of film distribution (dual inventories, e.g. releasing prints in multiple formats, are costly), incompatible sound-track formats are unlikely to get used in more than a few showcase theatres.
It is therefore an object of this invention to provide an improved motion picture optical sound-track format.
It is a further object that the improved format provide four discrete channels.
It is yet a further object that the improved format be compatible with both the existing matrixed two-channel SVA format and the standard mono photographic sound-track format.